The Mancubus (plural: Mancubi) is one of the few Doom 3 monsters that remain faithful to their original designs, albeit a little. It is greenish-grey in colour, rather than brown like the classic Doom version. Like the original version, the Mancubus is a very large, fat demon with a fireball launcher for each arm, but, unlike the original, it does not fire the two launchers simultaneously, firing each in turn instead. The Mancubus stands about 2 heads taller than a normal human, and its bulk is about 2 to 3 times as wide.
Compared to the classic Doom Mancubus, this one is more elephant or seal-like, or Cthulhu-like, since the Making of Doom 3 book states the latter. Their strange head was explained by the game designers. The central tentacle is a breathing tube, consisting of tiny holes along its sides (which are visible if viewed closely). Both side tentacles were actually supposed to be tusks, making them even more like elephants. Due to a graphical and model design glitch, they were reduced to feeding tools, and are stated to be used like the mouth of an octopus. It is unknown whether they have a beak inside their mouth or not.
The main differences between the Doom 3 Mancubus and the Doom II Mancubus is the skin color, the face, and the feet. In Doom II, the Mancubus had a mouth, leather-imitating colored skin, and elephant-like feet. In Doom 3, the Mancubus has a small "trunk" instead of a mouth, pale skin, and their feet look like fanned out pieces of flesh.
Mancubi are quite rare, and only appear in two levels in Doom 3. They first appear in a sort of "mini-boss" fight against 3 Mancubi in Recycling Sector 2 (possibly mirroring the Mancubus' introduction in Doom II's seventh level) and several are also encountered throughout Hell. They also appear in Resurrection of Evil, although again they are mostly confined to Hell. In its introductory cutscene, the hero enters the mentioned area when minor ground shakes and low-pitched moans start to take place. Suddenly, the creature crawls out from a waste duct, literally headbutting its way out, then stands up to fight the player.
The Doom 3 Mancubus has extremely high offensive capabilities. Though it can only fire one cannon at a time, unlike its classic counterpart, it more than makes up for this by being able to fire a steady barrage of powerful fireballs from each cannon, unlike the classic version, which can only fire three volleys from both cannons at once. It is also tougher than the classic version, taking 2 to 3 magazines of Machine Gun fire, a full magazine of Chain Gun or Plasma gun fire, or as many as six direct rocket hits to kill, making it the toughest (but also one of the slowest) non-boss creatures in the game.
Though it is very tough, it is also very slow. You can run and circle around it, then use the Shotgun to hit its back. It will turn to your direction and attack you, however, you can run the opposite way it turns to keep you facing its back (take example: if it turns right, run left) and keep firing the shotgun to finish it off. Areas with columns improve the effectiveness of this tactic.
Furthermore, unlike the classic Mancubus, the Doom 3 version has a melee attack: both of its cannons can be used as hammers, which, while attacking almost spontaneously, it is possible to use its long "recharge" time to actually kill the Mancubus with any melee weapon (Chainsaw being the best bet). However, melee combat is not advisable as the Mancubus' high HP makes it durable enough to withstand your chainsaw, not to mention its crushing melee smash is one of the most powerful enemy melee attacks in the game, second only to the Cyberdemon's stomp.
Mancubi are real powerhouses, with extremely high health and a powerful ranged attack with a reasonably high rate of fire. Players are fully justified in using their most high-powered weaponry to bring one down. The weapons that are effective against the Mancubus are the Chain Gun, the Rocket launcher, Plasma Gun, and the BFG. Grenades also work well due to the lumbering speed of these behemoths. Headshots are effective and quite easy to score on a Mancubus, since this creature is a slow-moving and very large target. A Mancubus can be killed with about 4 rocket launcher headshots. If the Soul Cube or Artifact is available, by all means, use it.
- If the Mancubus model is examined more closely (e.g. using the noclip cheat or a model viewer) it is possible to see hooks with suckers where the trunk begins, suggesting the trunks are actually feeding tools; when the Mancubus shakes its head, they flap around. This could indicate that the design of the Mancubus was inspired by Cthulhu, like much of Quake. The tentacles can also be seen in the beast's introduction FMV. The designer of the mancubus said that the Mancubus is not only inspired by Cthulhu but also of a shock site image of a man who took off his lower jaw from a gun shot. 
- Some of the original Mancubus grunts from Doom II were used in the Doom 3 version.
- The Vagary's transparent abdomen contains a malformed Mancubus fetus, leading to speculation that the Vagary is the "mother" of the Mancubus species (this could be technically incorrect).
- These monsters are usually accompanied by Cherubs in the original. This has lead to speculation they are the offspring of Mancubi. This is highly unlikely, since they look and act completely differently (just like the example above of a Vagary gestating a malformed Mancubus fetus), though it still may be possible. In the expansion, however, they don't appear alongside them anymore. The Mancubi are at one point by themselves instead of accompanying either Maggots or Hell Knights.
- There are only 10 Mancubi in Doom 3 (not counting expansions) : 3 in Recycling Sector (in "their" room) and 7 in Hell (one before the Berserker room, 4 on the hammering pillar room, one on the fiery large room after the Cherubs corridor, one behind the door in this room, and the last one in the corridor next the end of the level).